Agreement between GCRC Core Laboratory at UC San Diego and GCRC at University of Kentucky to perform catecholamine assay on samples sent to San Diego. Determine the potential role of autonomic dysfunction in sick sinus syndrome (SSS) by evaluating the autonomic system modulation of cardiovascular responses to tilt and isometric exercise in normal subjects and those with SSS. Determine the degree of cardiac abnormalities and their relationship to systolic function before, at 6 weeks and 3 months post-implant.